Electron tube voltage protection circuit



Jan. 27, 1953 A. A. BARCO ELECTRON TUBE VOLTAGE PROTECTIOII cmcun Filed Aug. 29, 1950 \kumvm 5% E R INVENTOR Patented Jan. 27, 1953 ELECTRON TUBE VOLTAGE PROTECTION CIRCUIT Allen A. Barco, Princeton, N. J assignor to Radio Corporation of America, a corporation of Delaware Application August 29, 1950, Serial No. 181,971

The present invention relates to protective circuit arrangements for electron discharge tubes which will prevent excessive voltage stresses from being applied between the discharge tube electrodes. It relates more particularly, although not necessarily exclusively, to alternating current and direct current voltage dividing arrangements for application to the heater and cathode elements of electron discharge tubes whereby to prevent excessive heater-cathode voltages from being developed without imposing excessive loading on signals appearing at the cathode.

More directly, the present invention, in one of its aspects, is concerned with an improved circuit arrangement for applying electron discharge tube damping devices of the heater-cathode variety in shunt with highly inductive circuit elements, such as, for example, electromagnetic cathode ray deflection yokes and associated transformer driving means.

In the design of electronic equipment, it is generally found necessary to exercise considerable care in the connection of the heaters and cathodes of indirectly-heated-cathode type electronic tubes in order to insure that the maximum permissible heater-cathode voltage of the particular tube in question is not exceeded. For example, it is common-place in the electronic art to supply the heaters of heater-cathode type tubes from a low voltage high current transformer winding having a center tap which is returned to a datum potential such as, for example, chassis ground. If then, a plurality of electron tubes thus supplied are connected in a D. C. amplifier fashion, it will be found that the cathodes of successive tubes in the D. C. amplifier must be operated at respectively higher and higher potentials relative to chassis ground. A limit will therefore eventually be reached to the number of stages which can be safely supplied from the same filament transformer. This limit will be defined by the maximum permissible heater-cathode Voltage rating of the tubes beyond which voltage break down will occur between the heater and cathode elements. For the more advanced stages of such an amplifier, other filament transformer windings, of course, must be used. Obviously these windings will not be grounded but will be maintained at some voltage level with respect to ground. This well known technique is, of course, limited by the maximum permissible secondary to primary voltage rating of the transformer. This latter limiting factor is also one of the reasons why heater-cathode potential problems can not always be solved .by merely connecting the heater with the cathode. A second reason why a mere connection of the heater with the cathode of the electron tube cannot always be allowed is that 6 Claims. (Cl. 315-27) by such a connection, any circuit capacity existing from the heater to ground is imposed on the cathode electrode. Depending upon the particular circuit this additional capacity may be intolerable.

The above discussed heater-cathode voltage problem is perhaps no more pronounced than in certain types of present day electromagnetic deflection systems for cathode ray tubes. In such deflection systems there is generally employed an electron discharge tube of the heater-cathode variety which is placed in shunt with electromagnetic deflection yoke, or a portion of the transformer driving the yoke, in order to cyclically damp and recover a part of the magnetic energy stored in the deflection yoke. The voltage peaks appearing across the deflection yoke are very high, sometimes upward of 1,000 volts. Since the anode of the damper tube is connected with one terminal of the deflection yoke while the cathode is connected with the other yoke terminal, it is frequently found that the cathode is forced to make rather high amplitude voltage excursions relative to chassis ground. The problem Of heater power supply for the damper tube therefore becomes of primary concern. If the heater is directly connected to the cathode then the transformer winding supplying the heater must also be capable of withstanding the high peak voltages relative to ground that appear on the cathode. If, on the other hand, the heater supply winding is conventionally grounded by bringing the center tap of the heater supply winding to the chassis, then the high peak voltages involved will impose an excessive stress between the heater and cathode of the tube.

Moreover, in electromagnetic deflection systems it is oftentimes desirable to keep the total circuit capacity appearing across the deflection yoke at a minimum in order to shorten the retrace time of the deflection cycle. However, if, as in the former instance, the heater and cathode are directly connected to reduce heater-cathode stress, then the entire circuit capacity from the heater supply winding to ground may be imposed in shunt with the deflection yoke.

It is, therefore, a purpose of the present invention to provide a novel circuit arrangement which offers a satisfactory solution to the above mentioned problems.

It is another object of the present invention to provide anovel voltage dividing arrangement that permits the reduction of heater-cathode voltage stress in heater-cathode type electron tubes without excessively loading down the electrical circuit connected with the discharge tube cathode.

It is further an object of the present invention to provide an improved heater supply system for electron discharge tube used as deflection damping devices in present day electromagnetic deflectioncircuits for cathode ray beam tubes.

In the realization of the above objects and features of advantage the present invention contemplates the use of a primary and a secondary voltage dividing system which is connected between the electron tube cathode and heater and in some instances, also including a connection from the discharge tube heater to chassis ground potential. The primary voltage dividing system is capacitive in nature and affords transient protection for the discharge tube by preventing instantaneous high peaks of voltage from causing excessive heater-cathode potentials. This primary voltage dividing circuit comprises the connection of a capacitor between the heater and cathode of a discharge tube. Assuming that the heater is supplied from a heater winding which is insulated above ground, the value of this capacitor is chosen in accordance with the total heater to ground capacity of the tube circuit so that the voltage appearing across the heater and cathode is less than the maximum permissible. If the cathode, during circuit operation, assumes some high static operating potential with respect to ground, a secondary or resistive voltage dividing circuit is then provided by the present invention. This comprises the connection of a resistor between the heater and of the amplifier I4 is connected to the tap IS on the upper winding of the auto transformer 18. The lower terminal .of the upper winding is in turn connected through inductance 2D and through capacitor 22 to the upper end of the horizontal deflection yoke winding 26. The lower end of the yoke winding is connected to a positive potential supply terminal indicated at 28. A capacitor is connected in shunt with the combination of inductance 29 and capacitor 22. The deflection yoke winding 26 is also connected in shunt with the lower winding 32 of the autotransformer I8. For purposes of power recovery damping of the deflection yoke 26, a diode 34 is shown connected from the right hand terminal of inductance 2D to the positive supply termiline.

cathode of a discharge tube followed by the connection of a second resistor between the heater and chassis ground. Again, the values of these resistors are chosen to keep the static D. C. potential appearing between the heater and cathode of the discharge tube below the maximum permissible.

A more complete understanding of the present invention as well as a realization of its other features of advantage thereof may be obtained through a reading of the following description especially when considered in connection with the accompanying drawing, which, by a-single figure, illustrates one embodiment of the present invention as applied to an electromagnetic beam deflection system of the television variety.

Turning now to the single figure, there is shown by way of example, a typical electromagnetic type power recovery deflection circuit for cathode ray beam deflection tubes. The particular circuit shown is merely exemplary of a wide variety of difierent types of deflection circuits which employ a heater-cathode type discharge tube for voltage damping purposes. of deflection circuits shown and the inherent advantages thereof form no part of the present invention but are fully disclosed and discussed in U. S. Patent No. 2,536,838, granted to Edwin L. Clark on January 2, 1951 entitled High Efiiciency Cathode Ray Beam Deflection System filed as U. S. Patent application Ser. No. 95,106 on May 24, 1949 and allowed June 16, 1950.

Notwithstanding the complete treatment of the general type of deflection circuit shown in the figure, by the above referenced U. S. Patent, a brief rsum of the circuit elements and their operation is expedient. There is shown at l9 some form of deflection wave form generator which may be of any convenient type such as, for example, a multivibrator or simple sawtooth discharge capacitor variety. The deflection Wave form developed by the generator it is illustrated at 12 and is applied to the input of the deflection output amplifier I l. The output The particular form nal 28. t

For purposes of illustrating the present invention, the diode 3 is shown to be of the indirectly heated cathode variety having a heater 35 supplied with power from a transformer secondary winding 38. The filament transformer 40 of which the winding 33 is a part, has a primary winding 44 indicated for connection with some source of alternating current power supply such as, for example, the to volt power supply The upper terminal of the upper autotransformer winding it is connected with a high voltage rectifier 46 which is adapted to provide a high unidirectional potential for the accelerating electrode 58 of the kinescope 50. The vertical deflection winding 5d of the deflection yoke will, of course, be appropriately supplied with deflection energy from a suitable deflection circuit (not shown).

The circuit thus far described forms no part of the present invention is fully discussed, as pointed out above, in the above referenced U. S. Patent by Edwin L. Clark. Its operation is briefly as follows. The output tube i4 is conductive only during the last half of the linearly rising portion of the saw-tooth wave form I2. Assuming there is some charge on the capacitors 22 and 30, this initiates a current rise through the deflection yoke 26 which is afforded a proper impedance match to the output tube 14 by merit of the connection with the low impedance winding 32. As soon as the linear rise of the saw-tooth ceases, conduction of the output tube [4 also ceases, and an oscillatory ringing occurs across the yoke 26 and, therefore, across the diode 34. This ringing continues for approximately one-half cycle, at which time a high negative potential peak is applied to the cathode ill of the diode 34. Conduction of the diode then damps the ringing of the yoke inductance 23 along with the inductance of the auto-transformer and causes current to pass through the diode in a direction such as to charge capacitors 22 and 10 up with the polarity indicated in the figure. This damping current is also linear in nature and constitutes the other portion of the deflection cycle during which tube It is non-conducting. The voltage across capacitors 22 and 30 is in aiding relation to the power supply potential at terminal 28 so as to increase the efficiency of the deflection circuit.

According to the present invention a capacitor 56 is connected between the cathode 3'! and the heater 35 of the discharge tube Be. This value or" capacitor 56 is chosen in accordance with the value of the circuit capacitance 58 existing between the heater power supply circuit and ground. Thus, when the high transient peak voltage occurs at the cathode 31 of the diode 34 due to the half cycle of the free ringing of the yoke 26, a voltage dividing action is invoked between the capacitors 56 and 58 so as to maintain the heater-cathode voltage below the maximum permissible value for the particular diode being used. If desired, the circuit capacity 58 may be supplemented by additional capacity and the value of capacitor 56 accordingly altered.

It is noted that had the heater and cathode of the diode 54 been merely connected together as discussed hereinabove, the circuit capacitance 58 would have been placed in shunt with the deflection yoke 26 and thereby decrease the resonant ringing frequency thereof. This would have increased the fly-back time of the deflection cycle which is generally undesirable. This effect is fully discussed in an article appearing in the December 1949 issue of the RCA Review by Otto H. Schade entitled Characteristics of High Efficiency Deflection and High Voltage Supply Systems for Kinescopes.

In further accordance with the present invention a resistor 60 is placed in shunt with the capacitor 56 and a companion resistor 62 is connected from the heater circuit of the diode 34 to ground. Resistor 62 may be connected from any point on the heater circuit, the particular point of connection shown in the figure being only exemplary. The use of these resistors is optional but provide a convenient means for stabilizing the direct current potential of the heater circuit relative to ground. This resistive voltage divider has been referred to above as the secondary voltage dividing system. Furthermore, should the direct current potential at the upper terminal of condenser 22 be in excess of the maximum permissible heater-cathode potential of the diode 34, resistive secondary-toground leakage of the transformer 40 might endanger the tube, notwithstanding the capacitive voltage dividing system discussed above. Thus, the values of resistor 60 and 62 may be made suificiently high so as not to load down the circuit but so related to one another that the direct current voltage drop appearing between the heater and cathode can never be in excess of the maximum heater-cathode potential rating of the diode.

From the above, it maybe seen that the applicant has provided a simple and inexpensive protection circuit for electron discharge tubes of the indirectly heater-cathode variety. Also, although the principles of the present invention are particularly useful in preventing excessive heatercathode voltage from being developed, it is obvious that the same fundamental technique may be applied to other electrodes of the discharge tube as occasion demands it. Furthermore, it is to be understood that the utility of the present invention is in no way limited to the particular environment of electromagnetic deflection circuits but that the present invention finds useful application to electronic circuits in general, wherever heater-cathode voltage ratings of discharge tubes are being otherwise approached or exceeded.

Having thus described my invention what is claimed is:

1. In an electrical circuit, the combination of, a. reference potential datum, a discharge tube having at least an anode, cathode and heater electrode, said tube having a predetermined naximum heater-cathode voltage rating, an output terminal at which appears a signal waveform relative to said datum having a peak voltige in excess of said heater-cathode voltage rating, a set of heater power supply terminals coupled with said heater for excitation thereof, said terminals having a predetermined circuit capacity relative to said datum whereby said heaters are given a like circuit capacity relative to said datum, a connection from said discharge tube cathode to said output terminal, and 3, capacitor connected from said cathode to said heater.

2. Apparatus according to claim 1 wherein the value of said capacitance is chosen in accordance with the value of said heater-datum capacitance value such as to form a voltage dividing circuit which establishes the A. C. voltage drop across said capacitor at a value below the heatercathode voltage rating of said tube;

3. Apparatus according to claim 2 wherein the output terminal is established at a high direct current potential relative to said datum and wherein there is additionally provided a first resistance in shunt with said capacitor and a second resistance connected between said heater and said datum, the value of said first and second resistances being so related that the drop across said first resistance is below the heater-cathode voltage rating of said discharge tube.

4. In an electromagnetic cathode ray deflection system, the combination of, a datum potential terminal, a deflection output terminal at which appears a deflection signal waveform designated for excitation of a cathode ray beam deflection yoke, means for referencing said output terminal with respect to said datum potential terminal, a damping discharge device having at least an anode, cathode and heater, connections placing said discharge device in damping relation with respect to said deflection output terminal, said connections including a path from said output terminal to said damping device cathode, a heater power supply transformer winding connected with said clamping device heater, said winding and its connections sustaining a predetermined value of circuit capacitance with respect to said datum potential terminal, and a capacitor connected from said cathode to said heater winding.

1 5. Apparatus according to claim 4 wherein the deflection signal waveform appearing at said damping device cathode defines voltage peaks relative to said datum terminal which exceed the maximum permissible heater-cathode voltage rating of said damping device and wherein the value of said capacitor is so chosen relative to the value of said circuit capacitance that the Voltage appearing thereacross is less than said maximum permissible heater-cathode voltage.

6. Apparatus according to claim 5 wherein there is additionally provided a first resistance in shunt with said capacitor and a second resistance connected between said heater winding and said datum terminal.

ALLEN A. BARCO.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,047,533 Von Ardenne July 14, 1936 2,078,666 Ka De11 Apr. 27, 1937 2,089,430 Roys et al Aug. 10, 1937 2,265,620 Bahring Dec. 9, 1941 2,440,418 Tourshou Apr. 27, 1948 2,536,838 Clark Jan. 2, 1951 

